Stimulated Brillouin scattering (SBS) is an important topic in the field of laser-drive inertial confinement fusion (ICF). SBS can reduce the laser-target energy coupling efficiency, spoil implosion symmetry, and degrade target gain. The full-aperture stimulated Brillouin backscattering energy fraction diagnosis systems on the main laser fusion devices do not measure the spatial distribution of stimulated Brillouin backscattering signals, which simplifies the diagnosis system but loses the detailed features of stimulated Brillouin backscattering signals. An angle-resolved full-aperture backscattering diagnosis system is developed for the Shenguang-Ⅱ Upgrade (SG-ⅡU) facility to study the SBS process in the integration experiment of the double-cone ignition (DCI) scheme. The evaluation reveals that the angular resolution of the system is about ±2 °, and the diagnostic accuracy of energy fraction is ±15%.

The design of the system is guided by the temporal characteristics of the backward reflection signals on the SG-ⅡU facility. It is found that both SBS signals from the target and the third harmonic reflected light from the triple-frequency-conversion crystal (called laser signals in this work) propagate along the same direction but with a fixed time delay dependent on the geometry of the final optical assembly. The level of laser signals is proportional to the incident laser energy and is stable so that it can be used as a reference to infer the SBS energy fraction. An optical fiber array is designed to collect the angle-resolved signals reflected from the servo mirror of the SG-Ⅱ U facility. Then, the signals are split into two beams and recorded with two intensified charge-coupled devices (ICCD) separately. The time delay between the two ICCDs is determined by that between SBS signals and laser signals. To make sure the ICCDs record accurate signal energy and verify the timing and intensity ratio between the SBS signals and laser signals, the temporal characteristics of the ICCDs gating process are tested, and the total signals are also recorded with a streak camera coupled with a spectrometer. Assuming that the level of laser signals is known, the SBS level can be inferred.

In this experiment, the related experiments under a laser focusing distance of 100 μm, 125 μm, and 150 μm from the crown are carried out to improve the laser-target energy coupling efficiency. Fig. 5(b) is the schematic diagram when the laser focusing distance is 100 μm. The distribution of stimulated Brillouin backscattering energy fraction under different laser focusing distances is investigated experimentally, and the related diagnostic results are shown in Fig. 7. For the cone target, when the laser focusing distance increases from 100 μm to 150 μm, the distribution of stimulated Brillouin backscattering energy fraction changes significantly. It can be seen from Fig. 5 (b) that for the cone target, when the laser focusing position is different, the local incident angle of the laser relative to the target surface varies, which causes the change in the laser-target energy coupling efficiency and the change in the plasma state during the laser loading process. Eventually, the distribution of stimulated Brillouin backscattering energy fraction also changes. For the diagnosis system in this paper, the main error sources are ICCD gain fluctuation, ICCD trigger jitter, ICCD gating characteristics, and data calculation errors. These factors are related to the specific experimental conditions and experimental settings. The comprehensive evaluation shows that the diagnostic accuracy of this diagnosis system is ±15%.

The SG-Ⅱ U facility is the main laser device for the research on the DCI scheme research at present. The first stage of the DCI scheme, quasi-isentropic compression, is completed by the 8-channel nanosecond beams. The laser plasma interaction process that occurs in this stage has a great influence on the compression effect. For the SG-Ⅱ U facility, a compact and portable stimulated Brillouin backscattering diagnosis system is developed on the basis of the diagnosis principle of relative measurement. The system can diagnose the distribution of stimulated Brillouin backscattering energy fraction with an angular resolution of about ±2°, and the diagnostic accuracy of energy fraction is ±15%. The preliminary diagnostic results show that the distribution of stimulated Brillouin backscattering energy fraction depends heavily on the focal location of the laser pulse on the target surface. The research provides reliable experimental results for the further study of SBS in the DCI scheme.